The Ultimate Guide to Create Spring Boot Custom Annotations

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A project in Spring Boot is littered with such annotations across the whole project.

But do you know what problems these annotations solve?

Why were custom annotations introduced to begin with?

How to create spring boot custom annotations?

Today, I will cover:

• Why create custom annotations?

• Key benefits of annotation

• How to create custom annotations?

• When to use custom annotations?

• When not to use custom annotations?

• What are the disadvantages of using custom annotations?

🎯 Why Create Custom Annotations?

In Spring Boot, annotations are more than just a way to add metadata. They

• Simplify complex tasks

• Reduce boiler-plate code

• Enhance code-readability

Before Spring introduced custom annotations, configurations like email validation were typically managed using XML configuration files.

The XML configuration would define beans, validators, and other necessary components to perform tasks such as validating email addresses.

Here's an example of how email validation might have been configured using XML in a Spring application:

As you can see, this can easily become a nightmare where there are hundreds of classes with many of them relying on each other.

It also meant a developer has to go look-up this XML every time they have to add a new dependency.

Key Benefits of Custom Annotations

Simplification of Configuration

Spring introduced custom annotations to simplify configuration by allowing developers to use annotations directly in their code.

This reduced the need for extensive XML configuration, making the codebase cleaner and easier to maintain.

Support for Declarative Programming

Custom annotations in Spring enable a declarative approach.

Developers can use annotations like @Transactional, @Cacheable, or @Scheduled to declare desired behaviors without writing the underlying logic.

This results in more readable and maintainable code.

Handling Cross-Cutting Concerns

Spring's custom annotations, often used in conjunction with Aspect-Oriented Programming (AOP), allow developers to handle cross-cutting concerns in a centralized manner.

For example, the @Transactional annotation can be used to manage transactions across multiple methods or classes without scattering transaction management logic throughout the code.

Reducing Boilerplate Code

It reduces the need for boilerplate code by encapsulating common behaviors.

For instance, the @Autowired annotation simplifies dependency injection, allowing Spring to automatically inject dependencies, rather than requiring explicit constructor or setter methods

Improving Code Readability and Consistency

By abstracting configuration and cross-cutting concerns into annotations, Spring improves the readability of the code.

You and your peer developers can quickly understand the purpose of a method or class by looking at its annotations, and annotations help enforce consistency across the codebase.

Framework Flexibility and Extensibility

Custom annotations allows developers to create their own annotations tailored to specific needs, thereby extending the framework's functionality in a standardized way.

This flexibility has helped Spring remain relevant and powerful across different types of applications and architectures.

🚀 How to Create a Custom Annotation

Step 1: Define the Annotation

• Create a new annotation by defining an interface.

• Use @interface to declare it.

• Add meta-annotations to specify how the annotation should behave.

• @Target: Indicates where the annotation can be used (e.g., methods, classes).

• @Retention: Indicates how long the annotation is retained (e.g., runtime, compile-time).

Step 2: Create an Aspect to Handle the Annotation

You can create a custom logic to process the annotation using Spring’s BeanPostProcessor, Aspect, or custom annotation processing logic.

Step 3: Apply the Annotation

Apply your custom annotation to methods, fields, or classes as defined

Use Cases Where Custom Annotations Are a Good Approach

Cross-Cutting Concerns

Custom annotations are ideal for handling cross-cutting concerns like logging, security, transaction management, and caching.

These are concerns that affect multiple parts of an application but are not related to the core business logic.

The @LogExecutionTime annotation above is a good example as that can be used across all the methods and it does not have any business logic.

Declarative Programming

When you want to specify what should happen rather than how it should happen, custom annotations provide a clean and expressive way to do this.

Annotations like @Cacheable or @Retry allow developers to enable caching or retry logic declaratively, without writing the implementation code manually.

Framework or Library Integration

Custom annotations can simplify the integration of frameworks or libraries by hiding the complexity behind an easy-to-use annotation.

Annotations like @Autowired in Spring help in injecting dependencies without having to manually instantiate them.

Encapsulation of Complex Logic

When complex logic needs to be encapsulated in a reusable way, custom annotations can provide a clean API for applying this logic.

An annotation like @RateLimit could encapsulate logic to limit the number of times a method can be called, without cluttering the method's body with this logic.

Use Cases Where Custom Annotations Should Not Be Used

Simple or One-Off Logic

If the logic is simple or only needs to be applied in a single place, creating a custom annotation is overkill and can unnecessarily complicate the code.

Logic That Requires Dynamic Behavior

Annotations are statically defined at compile-time, making them unsuitable for scenarios where behavior needs to be dynamically determined at runtime.

If a method's behavior should change based on user input or external configuration, handling this with custom annotations can lead to overly complex solutions.

Business Logic

Core business logic should not be abstracted into custom annotations, as this can make the logic less transparent and harder to maintain.

Using an annotation to encapsulate a business process like @ProcessOrder might hide important business rules, making the code harder to understand and maintain.

Complex Interactions Between Annotations

If the behavior depends on complex interactions between multiple annotations, it can lead to unexpected results and make the code difficult to understand and debug.

Combining multiple custom annotations that affect the same method (e.g., @Retry, @Cacheable, @LogExecutionTime) can result in unpredictable behavior and is difficult to manage

Performance-Critical Code

Custom annotations often rely on reflection or proxy mechanisms, which can introduce performance overhead. They should not be used in performance-critical sections of code.

Using a custom annotation to add logging to a method that is called millions of times in a tight loop could significantly degrade performance.

💡 TDLR - When to Use Custom Annotations

Custom annotations are perfect for handling cross-cutting concerns like logging, security, and transaction management.

They're also great for scenarios where you need to apply the same behavior across multiple parts of your application.

However, for simple, one-off logic, or where fine-grained control and flexibility are required, custom annotations might not be the best approach.

Consider the trade-offs before you decide to implement them.

🌟 Final Thoughts

Custom annotations are a powerful tool in your Spring Boot arsenal, but like any tool, they should be used judiciously.

They offer a clean, reusable way to handle repetitive tasks and enforce consistency across your codebase.

But be mindful of the potential downsides, especially when it comes to complexity and performance.

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Before You Go

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